(1) Field of the Invention
This invention relates to a metal hydride alkaline storage cell and a manufacturing method thereof, and more particularly to a metal hydride alkaline storage cell having an improved hydrogen-absorbing alloy surface and a manufacturing method thereof.
(2) Description of the Prior Art
The cell performance of a metal hydride alkaline storage cell employing hydrogen-absorbing alloy as a negative electrode active material largely depends on the degree of activation of the hydrogen-absorbing alloy therein. For this reason, a hydrogen-absorbing alloy used in this type of storage cells is pulverized into fine powder to enlarge a reaction area involved in the electrochemical reaction, to intensify the filling density in the electrode substrate, and thereby to enhance the energy density.
However, since hydrogen-absorbing alloy is a very active substance, it is corroded as a charge-discharge cycle is repeated, which results in a short cycle life.
In light of this problem, there has been suggested a hydrogen-absorbing alloy electrode wherein a metal (cobalt, nickel, and the like) oxide or hydroxide layer is formed on the surface of hydrogen-absorbing alloy powder.
Nevertheless, this type of hydrogen-absorbing alloy electrode still has such shortcomings that, when oxygen gas is generated in overcharging, the contact of the oxygen gas with the hydrogen-absorbing alloy is hindered by the metal oxide layer etc. on the surface of the hydrogen-absorbing alloy powder, which results in deterioration of absorbing performance of the oxygen gas and, consequently, high-rate charge characteristic.
In view of the above problems, Japanese Unexamined Patent Application No. 08-333603 discloses a cell wherein a coating film dotted with cobalt or the like is formed directly on the surface of the hydrogen-absorbing alloy powder without intermediary of an oxide.
In the cell according to the above-mentioned composition, however, there is no oxide formed on the surface of the powder, and the alloy surface therefore becomes flat and smooth with a small reaction area. Consequently, although cobalt or the like is dotted on the alloy surface, its effect as a catalyst cannot be sufficiently utilized.
The manufacturing method of the hydrogen-absorbing alloy with the above-mentioned composition comprises the steps of removing an oxide film on the hydrogen-absorbing alloy powder, then dotting the surface with cobalt or the like by dipping the powder into an aqueous solution containing metallic ions of cobalt or the like, and then washing the powder with water. In the washing process, however, cobalt or the like coated on the surface of the hydrogen-absorbing alloy is likely to be exfoliated and/or oxidized, and therefore cannot exhibit sufficient effect as a catalyst. Moreover, the manufacturing method of the above-mentioned invention requires such complex steps as removing an oxide film on the surface of the hydrogen absorbing alloy powder, washing, and the like, and consequently incurs high manufacturing cost.